The invention relates to a method for producing a micro system.
Thin films made of lead zirconate titanate (PZT) are widely used in micro system technology because of their advantageous physical properties, such as, for example, a high electromechanical coupling, a high dielectric constant or a high pyroelectric coefficient. A micro system typically has a substrate as a support for the thin film, where the substrate is usually made of silicon. The lead zirconate titanate is present in the thin film as a mixed crystal, which has a crystal symmetry that varies as a function of the zirconium content. Lead zirconate titanate with a low zirconium content is present primarily in the tetragonal phase, whereas lead zirconate titanate with a high zirconium content is present primarily in the rhombohedral phase. If the lead zirconate titanate has a morphotropic composition, for example, with a zirconium content of about 50%, then both tetragonal and rhombohedral microstructural constituents are simultaneously present in the thin film as grains. It is known to apply the thin film of lead zirconate titanate onto the substrate with a deposition method, in particular, a sputtering process, where the thin film is typically (111) textured. That is, the (111) directions of all of the grains of the thin film lie nearly parallel to the surface normal of the substrate surface.
In order to achieve the macroscopic piezoelectric and/or pyroelectric functionality of the thin film of lead zirconate titanate, it must have a preferential direction for the polarization. The optimal orientation of the preferred direction of the polarization depends on the respective desired physical effect that is to be achieved with the thin film, such as, for example, the pyroelectric effect. In order to optimize the pyroelectric effect, the preferential direction of the polarization of the thin film is oriented as parallel as possible to the surface normal of the substrate surface. Since in the rhombohedral phase the spontaneous polarization of a cell in the grid is in (111) direction, a (111) textured thin film provides the prerequisites for an optimal orientation of the polarization with respect to the pyroelectric effect.
The pyroelectric effect of the thin film is defined by the pyroelectric coefficient of the thin film. The value of the pyroelectric coefficient of the thin film depends more or less on the composition of the thin film made of lead zirconate titanate. If the thin film has a low zirconium content, then the thin film is present in a self-polarized form after its deposition and cooling down to room temperature. That is, during the deposition of the lead zirconate titanate the grown (111)-oriented thin film no longer changes its polarization state during the subsequent cooling. In contrast, the thin film that is rich in zirconium loses the self-polarization during cooling. This loss of self-polarization turns out to be disadvantageous in so far as for (111)-oriented, rhombohedral thin films of lead zirconate titanate a significantly higher pyroelectric effect is expected because of the optimum orientation of the polarization than for a thin film of a tetragonal composition. The production of self-polarized lead zirconate titanate is described by M. Schreiter, R. Bruchhaus, D. Pitzer and W. Wersing in “Sputtering of self-polarised PZT films for IR detector arrays”, Proceedings of ISAF XI, 1998.
If, for example, an infrared light sensor is constructed on the basis of a thin film of lead zirconate titanate, then the value of the pyroelectric coefficient of the thin film enters linearly into the intensity of the sensor output signal, so that in the course of satisfying the objective of a strong pyroelectric effect, a high sensitivity of the sensor can be achieved. Hence, only lead zirconate titanate (PZT) thin films that have a low zirconium content should be considered as infrared light sensors with a high sensor sensitivity.